Session: 01-03-01 Hydrodynamics

Paper Number: 79076

79076 - Experimental Investigation of Slamming Loads on Vertical Column Exposed to Short and Long Crested Waves 

Model test campaigns conducted recent years indicate large wave impact loads on vertical surfaces above the waterline for various types of moored floating structures. The impacts are caused by breaking or near breaking waves in extreme sea states, see e.g. Vestbøstad et al (2017).

 

It is common practice to assume long crested waves in design against slamming loads. This assumption is regarded as conservative, and there have been indications that the effect on impact loads could be significant.

 

I the present work an experimental investigation was set up to study the effect of short crested waves on the slamming loads against a rigid vertical column. A test model in scale 1:55 of a 31m diameter column at water depth 121m was studied. The area of the column facing the incoming wave was instrumented by 56 slamming panels in a 7x8 configuration. The setup covers 80 degrees of the cylinder circumference over a height of 24 meters from the still water surface and upwards. The force sensors used in the test have a very high natural frequency and the foundation for the sensors was made very compact and stiff to reduce structural dynamic effects. The sampling frequency used was 19.2 kHz.

 

In addition to the slamming loads measured on the column, shear force and bending moment at the base of the column were measured, and high-speed video recordings were made for slamming events above a given threshold. In front of the area instrumented by slamming panels a total of 9 wave probes were installed to monitor the profile of the wave events causing the impacts.

 

A sea state with Hs 13.5m and Tp 13.5s was selected in the tests. A total of 50 three-hour realizations were tested, both for short and long crested waves. This constitutes a solid basis for statistical analysis of extreme values and for comparing results from short and long crested waves. The standard deviation of spreading for the shorth crested waves tested was about 18-20 deg.

 

 

For all the three-hour realizations tested, wave elevation in the area close to the model was documented without the model in the basin. A wave probe located at the position of the column was used to document the spectrum of the sea state. To document spreading and steepness of the undisturbed wave close to the column, 14 wave probes were installed within a radius with 27.5m from the target location of the column.

 

In addition, wave probes up- and downstream to the test model, and transverse to the model, were installed. These probes were also present during the tests with model installed.

 

In this paper the test setup and test program are presented and experimental results from the investigation are presented and discussed.

 

 

 

References

 

Vestbøstad T.M., Økland O.D., Lian G and Stavang T.P. (2017). "Column Slamming Loads on a TLP from Steep and Breaking Waves", OMAE2017-61786, Proc of the 36th Int Conference on Ocean, Offshore and Artic Engineering, June 25-30, 2017, Trondheim, Norway.

 

 

Note: This paper was reviewed and approved for the 2020 conference (ASME Paper Number: OMAE2020‐18293), but was not published as the planned conference was cancelled due to COVID 19.

 

 

Presenting Author: Ole David Økland Sintef Ocean

Authors:

Ole David Økland Sintef Ocean
Gunnar Lian Equinor ASA
Tone Vestbøstad Equinor ASA